1. Field of the Invention
The present invention relates to a structure of a semiconductor device, and more particularly to a structure of a power semiconductor device used for power control,
2. Description of the Background Art
FIG. 15 is a sectional view schematically illustrating a structure of a power semiconductor device according to the background art. As shown in FIG. 15, the power semiconductor device according to the background art comprises a power chip 1 having a power element, a lead frame 20 made of a metal thin plate, a metal block 5 functioning as a heat sink for radiation and a mold resin 6.
The lead frame 20 has a die pad part 3 and an inner lead part 4. The power chip 1 is jointed to the die pad part 3 with solder 9 as a binder. An electrode (not shown) formed on the power chip 1 is connected to the inner lead part 4 of the lead frame 20 by an aluminum wire 8. The metal block 5 has a projection almost at its center and is arranged such that the projection faces the power chip 1 leaving a predetermined spacing with respect to a surface of the lead frame 20 opposite to the power chip 1. The mold resin 6 exposes a surface of the metal block 5 opposite to the lead frame 20 while sealing the power chip 1, the lead frame 20 and the metal block 5.
Attached to the exposed part of the metal block 5 is an external heat radiator 11. Part of the mold resin 6 present between the projection of the metal block 5 and the lead frame 20 is called a resin insulation layer 27.
In the power semiconductor device according to the background art, heat generated in the power chip 1 is emitted to the outside from the external heat radiator 11 through the lead frame 20, the resin insulation layer 27 and the metal block 5. The metal block 5 and the external heat radiator 11 made of aluminum or copper and have thermal conductivities of approximately 230 W/mK and approximately 390 W/mK, respectively. The lead frame 20, which is also made of metal such as copper, has a thermal conductivity of substantially the same degree as the metal block 5 and the external radiator 11. The resin forming the resin insulation layer 27 has a thermal conductivity of 1-3 W/mK. Thus, the resin insulation layer 27 has the thermal conductivity of substantially one hundredth that of any other material. This is a main factor that hinders thermal conduction.
Heat radiation properties of a semiconductor device are determined by the thickness and thermal conductivity of a material through which heat is conducted, an area of the material through which heat is conducted, and the like. The power semiconductor device according to the background art can achieve improved heat radiation properties by thinning the resin insulation layer 27 to thereby reduce part that has a low thermal conductivity through which heat is conducted. However, the resin insulation layer 27 needs an insulation breakdown voltage of several thousands of volts. This imposes limitations on its thickness to fall into the neighborhood of 0.5 mm, and improvements in the heat radiation properties are thus limited.
The thermal conductivity of the resin insulation layer 27 could be increased to as high as approximately 5 W/mK by using ceramic powder having a high thermal conductivity such as aluminum nitride powder or silicon nitride powder as a filler for the resin forming the resin insulation layer 27 to increase a filling factor. However, the resin insulation layer 27 is part of the mold resin 6, which causes the resin filled with ceramic powder to be used even for elements other than the resin insulation layer 27, i.e., elements that do not require high thermal conductivities. This results in a wasted use of expensive resin. In consequence, material costs of a semiconductor device are increased.
Heat generated in the power chip 1 is first conducted through the lead frame 20, and next, through the resin insulation layer 27. It is generally impossible to make the lead frame 20 thick in terms of processing unlike the metal block 5 or the like. Thus, the lead frame 20 has a thermal diffusion effect inferior to that of the metal block 5 or the like. This makes it difficult to fully extend an area of the resin insulation layer 27 through which heat is conducted, which has been a factor that imposes limitations on improvements in heat radiation properties.
It is an object of the present invention is to provide a semiconductor device with good heat radiation properties and good insulation breakdown voltages.
According to the present invention, the semiconductor device includes first a second semiconductor chips, first and second lead frames, a metal block, and resin. The first and second lead frames have one side main surfaces on which the first and second semiconductor chips are mounted, respectively. The metal block is provided on the other main surface of the first lead frame. The resin is formed to cover the first and second semiconductor chips, the first and second lead frames and the metal block. The first lead frame has a plurality of suspension lead parts projecting from the resin.
The present invention radiates well heat generated in the first semiconductor chip by means of heat radiation through the metal block provided on the other main surface of the first lead frame. At this time, the metal block, which is covered with resin, can maintain insulation relationship with the outside.
Further, the first lead frame, having the plurality of suspension lead parts, is brought into a state of a beam supported at two or more positions at least in the resin molding step, according to which its stiffness can be improved. This allows the resin covering the metal frame to be formed uniformly in thickness. As a result improved heat radiation properties can be obtained while securing insulation properties.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.